A.12 library(dcg/basics): Various general DCG utilities
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Availability::- use_module(library(dcg/basics)).
Source[semidet]alpha_to_lower(?C)//
Read a letter (class alpha) and return it as a lowercase letter. If C is instantiated and the DCG list is already bound, C must be lower and matches both a lower and uppercase letter. If the output list is unbound, its first element is bound to C. For example: 
?- alpha_to_lower(0'a, `AB`, R).
R = [66].
?- alpha_to_lower(C, `AB`, R).
C = 97, R = [66].
?- alpha_to_lower(0'a, L, R).
L = [97|R].